Update waf wrapper to print submodule help message

waf

@@ -2,15 +2,26 @@
 
 # Minimal waf script for projects that include waflib directly
 
-from waflib import Context, Scripting
-
+import sys
 import inspect
 import os
 
+try:
+    from waflib import Context, Scripting
+except Exception as e:
+    sys.stderr.write('error: Failed to import waf (%s)\n' % e)
+    if os.path.exists('.git'):
+        sys.stderr.write("Are submodules up to date? "
+                         "Try 'git submodule update --init --recursive'\n")
+
+    sys.exit(1)
+
+
 def main():
     script_path = os.path.abspath(inspect.getfile(inspect.getmodule(main)))
     project_path = os.path.dirname(os.path.realpath(script_path))
     Scripting.waf_entry_point(os.getcwd(), Context.WAFVERSION, project_path)
 
+
 if __name__ == '__main__':
     main()